The present invention relates to a sealed alkaline secondary battery with a cadmium negative electrode.
A typical example of an alkaline secondary battery is a nickel cadmium battery. Recently, there has been a strong demand for a battery with increased capacity due to size and weight reductions in electrical equipment. In order to meet the demand, a sintered electrode, which has been used as a negative electrode, has been replaced by a plastic bonded electrode. However, when repeatedly charged and discharged, a battery with a negative electrode is likely to be internally short-circuited earlier than a battery with a sintered negative electrode. The internal short-circuit problem is caused by a phenomenon known as "migration" in which cadmium active material grows and transfers from the negative electrode to the positive electrode. An nonwoven separator used in a sealed battery cannot prevent the internal short-circuit caused by the growth of the cadmium active material.
In order to suppress the migration of cadmium active material, a few methods have been proposed. The methods, and problems accompanying the methods, are briefly described as follows.
A first method is directed to adding an additive such as boric acid to the electrolyte to decrease the solubility of cadmium. The method suffers from a difficulty that, during charging and discharging, the polarization is increased, and therefore the discharge capacity is decreased.
A second method relates to mixing a large quantity of electrically conductive material such as nickel powder with the negative active material. The method has the following disadvantages. The energy density of the electrode is decreased. In addition, the method is not extremely effective in suppressing migration. For instance, the nominal capacity when the battery treated according to the second method is charged and discharged with a current of 1 C, its service life is no more than 1200 cycles. On the other hand, a battery with a sintered cadmium negative electrode can be charged and discharged at least 2000 cycles. Thus, the service life of the battery in question is about half that of the latter battery.
A third method teaches using a micro-porous separator having rectangular micro-pores. Cadmium active material cannot grow through the micro-porous separator, and therefore the method can substantially completely prevent the internal short-circuit caused by the growth of cadmium active material. However, the method is greatly disadvantageous in that it cannot be applied to sealed batteries. In a sealed battery, at the end of the charging period or when the battery is overcharged, oxygen gas produced from the positive electrode is absorbed through the reaction described by equation (1), whereby increased pressure in the battery is prevented. That is, the battery is maintained to be sealed. This function is based on the use of a so-called "open separator" EQU O.sub.2 +2H.sub.2 O+4e.fwdarw.4OH (1)
The micro-porous separator whose micro-pores are rectangular is extremely low in oxygen gas permeability. Therefore, with the micro-porous separator, oxygen gas produced at the end of the charging period or during over-charging has difficulty reaching the negative electrode. Accordingly, the oxygen gas absorbing reaction scarcely takes place with the negative electrode, so that the pressure in the battery is increased. Finally, the safety valve operates to reduce the electrolyte.
As is apparent from the above description, for the nickel cadmium battery using the plastic bonded cadmium negative electrode, a method has not been provided which is effective in preventing the internal short-circuiting caused by migration of cadmium active material.
On the other hand, a novel nickel cadmium battery has been proposed in which the charging of the negative electrode is accomplished before or simultaneously with the end of charging of the positive electrode. The novel nickel cadmium battery is designed so that, at the end of charging, the considerably large potential difference of the negative electrode is detected for control of the charging operation. That is, the novel nickel cadmium battery is a sealed battery which features higher capacity and a shorter charging time than the conventional nickel-cadmium battery. However, the novel nickel-cadmium battery is still disadvantageous because the potential difference between the positive electrode and the negative electrode is as high as 1.9 volts at the end of charging. As a result, the migration of cadmium active material is liable to occur, and therefore the internal short-circuiting may take place even earlier than in the conventional-nickel cadmium battery.
The battery is a sealed battery in which the over-charging is carried out for a short period of time. However, oxygen gas is produced from the positive electrode at the end of charging. In addition, oxygen gas is also easy to be produced by aging. Thus, the battery will produce a relatively large quantity of oxygen gas. Accordingly, when using the rectangular micro-pore type micro-porous separator, the oxygen gas thus produced cannot be absorbed with the negative electrode.
Thus, there has been a strong demand for a method which prevents the occurrence of the internal short-circuiting due to the growth of cadmium active material, and which maintains the oxygen gas absorbing function substantially unchanged.